Monitoring cameras are used in various scenarios, both indoors and outdoors. In many situations, it is valuable to be able to quickly focus the camera so that sharp and useful images can be provided depicting any objects of interest in a scene.
Automatic focusing, autofocus or AF, methods used to set focus of monitoring cameras may be divided into two main groups: passive AF methods and active AF methods. Some common passive AF methods are based on analysing the contrast of images captured by the camera for different settings of the focus lens system, and then selecting a focus position that gives the highest (or at least a high) value of contrast. There also exist other passive AF methods which are based on detecting phase differences, e.g., using specially designed focus pixels.
Active AF methods are based on performing a distance measurement to an object in the monitored scene using a range metering device, such as a radar or a laser range meter. The camera is set to focus on the distance measured by the range meter. Active AF methods are usually much faster than passive, contrast-based AF methods, and they also work better in a low-light environment where the contrast of the images is lower.
If a laser range meter is used, a low impact laser operating in the infrared, IR, spectrum is a common choice, providing an energy efficient and safe option. Since monitoring cameras already are provided with a filter for filtering out IR light, a so called IR-cut filter, the laser can be used without being visible in the images captured by the camera.
Some monitoring cameras offer both day and night functionality, such as those designed to be used in outdoor installations or in indoor environments with poor lighting. When in day mode, the camera IR-cut filter filters out IR light such that it does not distort the colours of the images as the human eye sees them, but when the camera is in night mode, the IR-cut filter is removed, thereby allowing the light sensitivity of the camera to reach down to 0.001 lux or lower. Near-infrared light, which spans from 700 nm up to about 1000 nm, is beyond what the human eye can see, but most camera sensors can detect it and make use of it.
Thus, when in day mode, i.e., when the light in the scene is above a certain level, the camera delivers colour images. As light diminishes below the certain level, the camera switches to night mode to make use of near infrared (IR) light to deliver high-quality black and white images.
However, when a camera with day and night functionality is equipped with active AF using an IR laser range meter, there will be a problem that the light from the IR laser is visible in the images when the IR cut filter is removed. This is easily solved by deactivating the IR laser meter when the camera is operating in night mode. However, it would be desirable to solve this problem in a different manner, especially since, as mentioned above, passive AF methods based on contrast measurement are less efficient in low light environments.